OTB-1 (Orbital Test Bed-1) minisatellite mission of SST-US with DSAC hosted payload
After working with a wide range of potential users, Surrey has finalized the OTB-1 flight manifest. The OTB-1 satellite will carry five demonstration payloads for a variety of commercial, government, and academic organizations. With the assistance of the Department of Defense STP (Space Test Program), OTB is scheduled to launch on the SpaceX Falcon Heavy. The OTB-1 satellite is the first true commercial all hosting satellite and the first spacecraft to be integrated at SST-US (Surrey Satellite Technology-US LLC), the new Surrey US facility in Englewood, Colorado. 1)
Hosted payload missions play a critical role in the development of new aerospace technologies. The OTB-1 satellite provides aerospace organizations with a low-risk opportunity to test new subsystems and payload technologies on an actual low-Earth orbit mission while sharing the cost of development and launch. The rideshare concept behind OTB-1 is a cost-effective way to rapidly space-qualify new equipment and generate in-orbit data.
The primary hosted payload of OTB-1 is DSAC (Deep Space Atomic Clock) of NASA/JPL (Jet Propulsion Laboratory), Pasadena, CA. DSAC is a miniaturized, ultra-precise mercury-ion atomic clock that is orders of magnitude more stable than current spaceborne navigation clocks. 2)
• Already in 2012, JPL inquired about a hosting position for their DSAC instrument. DSAC had recently lost its hosted position on IRIDIUM and was increasingly looking for hosting opportunities as the payload was maturing and ready for a flight demonstration needed to increase its TRL (Technology Readiness Level) from 5 to 7 (Ref. 7).
• In July 2013, SST-US announced that NASA/JPL had selected SST-US for the flight of the Deep Space Atomic Clock (DSAC) payload under the sponsorship of NASA's STMD (Space Technology Mission Directorate). DSAC will fly on the SST-US-owned-and-operated OTB (Orbital Test Bed) satellite. 3)
- Under the agreement SST-US will provide a hosted payload flight opportunity for the NASA DSAC payload on its upcoming OTB mission, scheduled for launch during 2015. DSAC features a miniaturized, ultra-precise mercury-ion atomic clock which is an order of magnitude more stable than today's best space-based navigation clocks. In-orbit demonstration of the precision timing and navigation capabilities of the DSAC instrument is a key requirement for NASA's pursuit of deep space exploration missions which require higher-precision data collection and autonomous radio navigation for time-critical events such as orbit insertion or landing.
• In July 2014, the SST-US has been awarded a firm-fixed-price IDIQ (Indefinite-Delivery/Indefinite-Quantity) contract under the HoPS (Hosted Payload Solutions) program from the U.S. Air Force SMC (Space and Missile Systems Center) Contracting Directorate. 4)
- SMC qualified SST-US as a vendor to meet the government's needs for various hosted payload missions. The HoPS IDIQ contract will provide a rapid and flexible means for the government to acquire commercial hosting capabilities for government payloads.
- Within the HoPS IDIQ scope, SST-US will provide hosted payload studies and hosted payload missions. The HoPS studies include those study activities related to enabling hosted payloads. The missions will encompass fully-functioning on-orbit hosted payload space and ground systems for government-furnished payloads on commercial platforms. In addition to the space and ground systems, the HoPS missions will include related on-orbit support for data transfer from the hosted payload to the government end user(s).
Owned and operated by Surrey US, the OTB-1 satellite is based on the CFESat (Cibola Flight Experiment Satellite) modular design developed by the Surrey group for LANL (Los Alamos National Laboratory) using the flight-proven SSTL-150 satellite bus. The Surrey engineering team in Colorado will utilize the same satellite integration methodologies developed by the Surrey group over a period of three decades that have resulted in 41 successful operational and experimental satellite missions. The nominal OTB spacecraft configuration and on-orbit orientation are shown in Figure NO TAG#. 5) 6) 7)
Figure 1: Illustration of the OTB-1 minisatellite and some components (image credit: SST-US)
Figure 2: Inverted illustration of the OTB-1 minisatellite (image credit: SST-US)
Figure 3: OTB-1 minisatellite scheme with accommodation of some payloads (image credit: SST-US)
Figure 4: OTB architecture based on 150 platform (image credit: SST-US, Ref. 7)
The OTB spacecraft has been designed for nadir pointing. The DSAC space-pointing requirements for the GPS antenna to be zenith-oriented represent a nice complement to that primary attitude (Figure NO TAG#). Some other key characteristics of OTB as they relate to DSAC on-orbit investigation follow:
Attitude knowledge and control: The spacecraft provides a relatively stable platform for the DSAC investigation. The spacecraft has no active propulsion system. All deployables (such as the solar arrays) are fixed once deployed. Deorbit at the end of the OTB mission is accomplished by a sail. The ADCS (Attitude Determination and Control Subsystem ) utilizes wheels, gyroscopes, and a magnetic control system utilizing magnetorquers and a magnetometer. Magnetic effects on the clock, from the magnetorquers and the Earth's magnetic field, have been modeled and are not expected to be a significant error source for DSAC's long-term stability. Twice during the one-year mission, the spacecraft rotates by 180º about the Z-axis (yaw flip) to maximize the solar array exposure and to minimize the exposure of the spacecraft radiator to the sun in response to seasonal variations in the environment. The clock is expected to operate through these maneuvers. Attitude knowledge is achieved via the magnetometer, gyroscopes, and the four sun sensors. The DSAC team receives the vehicle's attitude history via the same file-based transfer interface used for the DSAC payload and GPS telemetry. This data is available within approximately twenty-four hours of the payload and GPS telemetry.
TCS (Thermal Control Subsystem): While the spacecraft will experience significant temperature variations throughout the year-long mission and more modest variations during any given orbit, the thermal interfaces to the USO Ultra Stable Oscillator, clock, and GPS receiver have been designed so that the expected thermal variations meet their thermal operational range (with potentially short outages during the peak heating season). There are no specific requirements on thermal control of the cabling between the units or the GPS antenna. A particular thermal sensitivity relevant to the validation system is the GPS receiver's phase sensitivity to temperature variations.
Power: Spacecraft power is provided by the solar panels. The orientation of the solar panels has been selected to minimize multi-path effects on the GPS antenna, and their configuration is considered in the spacecraft model used by the precision clock determination process. The DSAC payload is required to operate across a wide input voltage range, but DSAC power converters, that manage the voltage beyond the interface, regulate the applied voltage sufficiently that changes in the input voltage are not expected to be a factor in PCD.
Incorporated into the satellite power system are solar cells designed by EMCORE Corp. of Albuquerque, NM, that are more efficient than those flown on previous Surrey missions.
Time: The OTB clock (not DSAC) is used to time-tag all platform telemetry (such as the attitude history). The OTB ground system will synchronize the on-board clock with a ground-based reference nominally once per day, and the ground is synched to UTC using a GPS receiver and a time server. The on-board computer sends that time out periodically to the various spacecraft nodes and other payload units that require it. The inaccuracies in these time tags relative to GPS time are small and introduce second order effects in the DSAC data analysis. (Note that the GPS receiver data is time-tagged using the GPS receiver clock). Any discontinuities in the telemetry time-tags with GPS time will be resolved by the DSAC ground system.
Figure 5 illustrates the on-orbit mission architecture, where the plan is to collect measurements between GPS and the host spacecraft using a GPS receiver connected to DSAC. The data will be processed on the ground by the DSAC team to validate the clock's in-space performance.
A key objective of the project is to demonstrate DSAC's performance and operability in space, and develop the technology to a point where it can be transitioned to industry for commercial production. DSAC's space demonstration will have proven its flight worthiness and will radically reduce risk to future missions that adopt it.
Figure 5: Overview of the OTB-1/DSAC mission architecture (image credit: NASA/JPL)
GN&C (Guidance Navigation and Control) changes and more:
The rideshare launch opportunity carried with it a specified and perhaps disadvantageous orbit of 720 km and 24º inclination. This put constraints on the TT&C (Tracking Telemetry and Control) and power as most missions, this bus was originally designed for are sun synchronous and most Surrey ground stations are at high latitudes. A yaw maneuver will also be required every 6 months in this orbit. Propulsion has been removed as orbit maintenance will not be required.
Figure 6: The 24º inclination orbit and associated beta angles chart (image credit: SST-US, JPL)
For the lower inclination, the limitation of a 5 minute TT&C pass per day, and the accommodations needed for the payloads has driven additional changes to the GN&C. There has been power negative spins identified that have to be avoided upon launch tip off and possible loss of control and communications through life. The following are some of the avionics changed or added:
• STIM-210 gyros (Sensonor Technologies) for pointing during eclipse
• Two ASMs (Active Safety Monitors)
• Moog Bradford sun sensors
The Active Safety Monitors are used in the case of an anomaly and will control OTB-1. The watch dog timer going off will turn these units on with redundant unit cold. Only the ASMs, AIM, X and Y wheels and heaters will be powered to regain safe mode. Given the removal of the standard star trackers and the above modifications, OTB-1 has the control specifications as shown in Table 1.
Table 1: OTB-1 control specification
GPS (Global Positioning System) receiver: The needs of DSAC required upgrading the typical SGR-10 GPS receiver with the TRIG-POD, which became part of the DSAC payload. Also, the GPS antenna with choke rings (to minimize multipath) has been mounted to the anti-nadir deck to give a maximum number of GPS satellites in view.
Figure 7: Undeployed OTB-1 showing GPS antenna and middle payload bay on right for DSAC modules and TRIG-POD (image credit: SST-US)
Magnetorquers: DSAC also needed a stable magnetic field environment and is shielded for this purpose, but the magnetorquer induced fields still required studied to make sure they were below the sensitivity levels of the payload. With the flexibility of the layout the magnetorquers were moved away from the atomic clock and an analysis was done to show that the simulated fields were tolerable.
Figure 8: Magnetic field simulation (image credit: SST-US)
Other changes: The reaction wheels were taken from 4 to 3. Maneuvers and safe modes can be accomplished with 2 wheels and torque rods. The wheels also have a great on orbit heritage and this saves space. Propulsion was eliminated because of the relatively high orbit and no need to maintain it. On the other hand a deorbit device will be employed to make sure the bus will come down in the required time frame (25 years). The relatively small demand for downlink allowed Surrey to drop the X- band system and use a S-band system including the new High Rate system being tested on-board.
• April 23, 2018: General Atomics Electromagnetic Systems (GA-EMS) announced today that it has completed full system and "ready for launch" pre-flight testing of its Orbital Test Bed (OTB) satellite. OTB will launch as part of the U.S. Air Force's Space Technology Program (STP-2) flight on the SpaceX Falcon Heavy rocket. The OTB hosts multiple payloads on a single platform for on-orbit technology demonstration. Among the hosted payloads on OTB is NASA's Space Technology Mission Directorate's DSAC (Deep Space Atomic Clock), designed and built at NASA's Jet Propulsion Laboratory, which supports deep space navigation and exploration. 8)
- "The completion of system testing marks a significant milestone, allowing us to effectively "button up" the OTB satellite in anticipation of delivery to Cape Canaveral for launch into space," stated Scott Forney, president of GA-EMS. "We believe OTB is a new paradigm in hosted payload satellite design and is paving the way to make space more affordable and accessible to customers looking to demonstrate and validate their technologies on-orbit."
• In November 2017, General Atomics of San Diego announced that it had acquired the majority of the assets of Surrey Satellite Technology US LLC (Surrey-US), an Englewood, Colorado-based provider of innovative small satellite technologies, systems and services. The assets and workforce will be integrated into General Atomics' Electromagnetic Systems Group (GA-EMS) to support the organization's growth initiatives focused on the development and delivery of small satellite and advanced payload systems. 9)
- "We look forward to the Surrey-US team joining us as we continue to expand our portfolio of small satellite and mission-support capabilities," stated Scott Forney, president of GA-EMS. "The Surrey-US team's expertise in designing and deploying small satellites and end-to-end mission solutions is complementary to ours. We are excited that GA-EMS now will offer ground-to-space solutions utilizing nano, micro and small satellites from 3U to 500 kg with a high degree of modularity and payload flexibility.
- Surrey-US was established in 2008 as a wholly-owned subsidiary of UK-based Surrey Satellite Technology Ltd. (SSTL) to address the U.S. market and its core capabilities include the complete design, manufacture, launch and operation of small satellites. SSTL is a world-renowned leader in the design, development and deployment of small satellites, with a 35 year legacy of successful launches, and a proven track record in pioneering and delivery of small satellite and complete mission solutions for defense, telecommunications, earth observation, and technology demonstrations.
- GA-EMS is a global leader in the research, design, and manufacture of first-of-a-kind electromagnetic and electric power generation systems. GA-EMS' research, development and technology innovation has led to an expanding portfolio of specialized products and integrated system solutions supporting aviation, space systems and satellites, missile defense, power and energy, and processing and monitoring applications for critical defense, industrial, and commercial customers worldwide.
Launch: A launch of OTB-1 as a secondary payload was scheduled for June 2018 on the SpaceX Falcon Heavy vehicle from Cape Canaveral, FL as part of the STP-2 (Space Test Program-2) launch manifest with the assistance of DoD. 10) — However, the STP-2 launch slipped due to ongoing SpaceX qualification testing and engineering review by both SpaceX and the Air Force. According to the "Upcoming Rocket Launch List," of 18 September 2018, a launch of STP-2 on SpaceX Falcon Heavy can be expected in early 2019. 11)
The OTB-1 (Orbital Test Bed-1) spacecraft is an ESPA (EELV Secondary Payload Adapter) compatible spacecraft that is a secondary payload to the primary mission of the FormoSat-7/COSMIC-2 program, manifested on the US Air Force STP (Space Technology Program) II rocket, a SpaceX (Space Exploration Technology) Falcon Heavy vehicle. The launch site is the LC-39A (Launch Complex-39A) at Cape Canaveral, FL.
The OTB-1 minisatellite mission is based on the 150 ESPA (Evolved Expendable Launch Vehicle) Secondary Payload Adapter) — a multiple launch system for small satellites used in the STP (Space Test Program) of the USAF . The most noticeable change is the addition of deployable solar arrays made necessary by the power demands and change in orbit inclination.
Figure 9: Illustration of the OTB-1 minisatellite in its launch position (right) on the ESPA ring (middle) and in its deployed configuration (image credit: SST-US, Ref. 7)
The secondary payloads on these flights are:
• OTB-1 (Orbital Test Bed-1) minisatellite mission of SST-US (Surrey Satellite Technology US LLC).
• DSX (Demonstration and Science Experiments) mission of AFRL
• GPIM (Green Propellant Infusion Mission), a demonstration microsatellite of NASA. 12)
• FalconSat-7, a 3U CubeSat mission developed by the Cadets of the U.S. Air Force Academy (USAFA) at Colorado Springs, CO.
• NPSat-1 (Naval Postgraduate School Satellite-1) of the Naval Postgraduate School, Monterey, CA
• OCULUS-ASR (Attitude and Shape Recognition), a microsatellite of MTU (Michigan Technological University), Houghton, MI, USA.
• Prox-1, a microsatellite of SSDL (Space Systems Design Laboratory) at Georgia Tech.
• LightSail-B of the Planetary Society, a nanosatellite (3U CubeSat) will be deployed from the parent satellite Prox-1.
• ARMADILLO of UTA (University of Texas at Austin), a nanosatellite (3U CubeSat) of ~ 4 kg.
• TBEx (Tandem Beacon Experiment), a tandem pair (3U CubeSats) of SRI International.
• Prometheus-2.1 to -2.8, seven 1.5U CubeSats (each of 2 kg) of LANL (Los Alamos National Laboratory).
• TEPCE (Tether Electrodynamics Propulsion CubeSat Experiment), a 3U CubeSat (3 kg) of NPS (Naval Postgraduate School).
• CP-9 , a joint CP-9/StrangeSat experiment, which is a collaboration between PolySat at Cal Poly and the Merritt Island High School, and is sponsored by the NASA LSP (Launch Services Program). CP-9 is a 2U CubeSat while StrangeSat is a 1U CubeSat.
• First launch: Six FormoSat-7 satellites (primary payloads) will be positioned in a low inclination orbit at a nominal altitude of ~520-550 km with an inclination of 24º. The parking orbit is at 720 km. Through constellation deployment, they will be placed into 6 orbital planes with 60º separation.
• Second launch: Six FormoSat-7 satellites (primary payloads) will be positioned in a high inclination orbit at a nominal altitude of ~720 km with an inclination of 72º. Through constellation deployment, they will be placed into 6 orbital planes with 30º separation.
Sensor complement (DSAC, ETB, MSA, iMESA-R, CUSP)
In addition to DSAC, OTB-1 will carry a Surrey payload suite comprised of the Electronics Test Bed, the FlexRX receiver, and the RadMon sensor. Developed by Surrey University in the UK, the Electronics Test Bed will evaluate and demonstrate a range of new electronic components, processors, and memory devices, enabling in-orbit heritage to be gained on components that may be incorporated into future designs. 13)
DSAC (Deep Space Atomic Clock)
Some background: In 2012, DSAC is a NASA technology demonstration mission with the goal to validate a miniaturized, ultra-precise mercury-ion atomic clock that is 100 times more stable than today's best navigation clocks. The DSAC project is sponsored by the NASA Space Technology Mission Directorate and managed by NASA(JPL (Jet Propulsion Laboratory) in Pasadena, CA.
On June 27, 2013, NASA/JPL selected SST-US LLC for the flight of the DSAC (Deep Space Atomic Clock) payload under the sponsorship of NASA's STMD (Space Technology Mission Directorate). DSAC will fly on the SST-US-owned-and-operated OTB (Orbital Test Bed) satellite. 14)
Over the past 20 years, NASA/JPL engineers have been steadily improving and miniaturizing the mercury-ion trap atomic clock, preparing it to operate in the harsh environment of deep space. In the laboratory setting, the Deep Space Atomic Clock's precision has been refined to permit drift of no more than 1 nanosecond in 10 days. 15) 16) 17) 18) 19)
Benefits of the Clock to Navigation and Science: In today's 2-way navigation architecture, the Earth ground network tracks a user spacecraft and then a ground-based team performs navigation. Relative to this, a 1-way navigation architecture can deliver more data with better accuracy, and is enabling for future autonomous radio navigation. Some specific examples of how a 1-Way deep space tracking architecture can benefit NASA include: 20) 21)
1) The DSN can support multiple downlinks on a single antenna, called MSPA (Multiple Spacecraft Per Aperture). Since the DSAC enabled 1-way radiometric tracking doesn't require an uplink, it can take full advantage of MSPA. For instance, at Mars two spacecraft equipped with DSAC can be tracked simultaneously on the downlink by a single antenna, while, with the current 2-way tracking capability, those two spacecraft must split their time on the uplink.
2) Deep space users with DSAC can utilize full view periods for tracking, unlike 2-way tracking, which reduces the available tracking time of the view period by the round trip light time. As an example, Cassini's Northern hemisphere view periods at Goldstone and Madrid are on the order of 11 hrs, so a round trip light time in the 4 – 5 hr range yields an effective ~ 6 hr 2-way pass. A 1-way pass using DSAC can utilize the full view period of 11 hrs, a near doubling of the usable data without needing to transition into a complicated 3-way tracking operation across multiple ground stations.
3) For outer planet missions, solar corona plasma effects are a frequency-dependent error source that dominates over other measurement errors and affects radiometric tracking across both short and long time scales. Use of a Ka-only 1-way downlink reduces these effects by 10 times relative to the typical user on an X-up/Ka-down 2-way paradigm. Indeed the Europa Flyby Mission's gravity science results needed for determining the presence of an ocean underneath the crustal ice requires Ka-band tracking, which is enabled via having DSAC.
4) Planetary atmosphere investigations using radio occultations can benefit from DSAC as well. Compared to today's radio occultations that rely on 1-way tracking derived using ultra stable oscillators, DSAC enabled measurements are upwards of 10 times more accurate on the time scales relevant to these experiments (that is, the several minutes that a spacecraft radio signal to Earth rises and sets as it passes through the atmosphere of interest before being occulted by the planet).
5) A 1-way uplink received by a DSAC-enabled spacecraft with a properly configured and capable on-board navigation system is able to self-navigate in deep space. Autonomous deep space navigation has been demonstrated using optical navigation with the DS1 (Deep Space 1) and Deep Impact missions. However, a complete implementation of a fully autonomous on-board navigation system would couple a DSAC-enabled 1-way forward radiometric tracking system with optical tracking from a camera system. This would combine the strengths of radio navigation for determining absolute location in deep space and in planetary orbit with the target relative navigation provided by the optical system. A combined 1-way radiometric and optical autonomous navigation system would provide a powerful solution for robotic missions where ground-in-the-loop operations are impossible (deep space encounters, planetary capture, real-time orbital operations, etc.), as well as supporting human exploration missions beyond LEO (Low Earth Orbit) that require crewed operations without ground support.
Figure 10: Comparing today's two-way DSN tracking architecture with a possible future one-way tracking architecture using DSAC (image credit: NASA/JPL)
Once in LEO, DSAC's spaceborne performance will be characterized via a year-long demonstration.
DSAC instrument overview:
The DSAC project will advance the 199Hg+ atomic space clock technology from a TRL (Technology Readiness Level) 5 to TRL 7 by developing an advanced prototype, verifying its performance in space, and demonstrating its viability as navigation hardware. The DSAC technology uses the property of mercury ions' hyperfine transition frequency at 40.50 GHz to effectively "steer" the frequency output of a quartz oscillator to a near-constant value. DSAC does this by confining the mercury ions with electric fields in a trap (Figure 1) and protecting them by applying magnetic fields and shielding. This provides a stable environment for measuring the hyperfine transition very accurately and minimizes sensitivity to temperature and magnetic variations. Coupling this with the fact that the DSAC technology has almost no expendables enables development of a clock suitable for very long-duration space missions. 22)
The current best estimate (December 2013) of the DSAC Demonstration Unit's (DU) size, mass, and average power are: 29 cm x 26 cm x 23 cm, 17.5 kg, and 44 W, respectively. The preliminary configuration of the DU is shown in Figures 1 and 12. Another project objective is to identify improvements needed for a flight version of DSAC so that it can be readily used by a future deep space mission or GNSS application. In particular, DSAC elements amenable to reductions in size, mass, and power.
Figure 11: Drawing of the DSAC mercury-ion trap showing the traps and the titanium vacuum tube that confine the ions(image credit: NASA/JPL)
Legend to Figure 11: The quadrupole trap is where the hyperfine transition is optically measured and the multipole trap is where the ions are "interrogated" by a microwave signal via a waveguide from the quartz oscillator.
Ground-based atomic clocks are the cornerstone of spacecraft navigation for most deep space missions because of their use in forming precision two-way coherent Doppler and range measurements. DSAC will deliver the same stability and accuracy for spacecraft exploring the solar system by providing an equivalent capability on-board a spacecraft for forming precision one-way radiometric tracking data (i.e., range, Doppler, and phase). This new capability could forever change the way we conduct deep-space navigation — by eliminating the need to "turn signals around" for tracking. Much the same way modern GPS (Global Positioning Systems) use one-way signals to enable terrestrial navigation services, the DSAC will provide the same capability in deep-space navigation — with such extreme accuracy that researchers will be required to carefully account for the effects of relativity, or the relative motion of an observer and observed objected, as impacted by gravity, space and time (clocks in GPS-based satellite, for example, must be corrected to account for this effect, or their navigational fixes begin to drift). 23)
DSAC's stability, as measured by its AD (Allan Deviation), is expected to be less than 3 x 10-15 at one day, with a ground laboratory version of DSAC currently demonstrating an AD of approximately 1 x 10 -15 at one day. Such a small spacecraft clock error will enable one-way radiometric tracking data with accuracy equivalent to or better than current two-way tracking data, allowing a shift to a more efficient and flexible one-way deep space navigation architecture.
The DSAC team at JPL is preparing a miniaturized, low-mass atomic clock — orders of magnitude smaller, lighter and more stable than any other atomic clock flown in space — for a test flight in LEO (Low Earth Orbit). The clock will make use of GPS signals to demonstrate precision orbit determination and confirm its performance, promising new savings on mission operations costs, delivering more science data and enabling further development of deep-space autonomous radio navigation.
The DSAC demonstration unit (Figure 12) and payload will be hosted on a spacecraft provided by Surrey Satellite Technologies US LLC, Englewood, CO. It will launch in 2015 into Earth orbit. The DSAC payload will be operated for at least a year to demonstrate its functionality and utility for one-way-based navigation.
Figure 12: Preliminary configuration of the DSAC Demonstration Unit (DU) with the mercury ion (199Hg+) trap configuration shown to the right (image credit: NASA/JPL)
The mission requirement is to validate the clock's stability and drift with an AD of at most 2.0 x 10-14 at one day while in orbit. This requirement is looser than the expected performance of the clock because of the presence of systematic errors in the measurement system; however, the performance of the validation system (like the clock itself) is expected to be better than this required level.
The DSAC payload consists of three subsystems (and associated cabling):
• An ovenized crystal USO (Ultra Stable Oscillator) produced by FEI (Frequency Electronics Incorporated) with short term (between 1 – 100 seconds) stability of at most 2 x 10 -13 and frequency drift below 1 x 10 -10/day
• The DSAC DU (Demonstration Unit)
• The GPS system is comprised of a JPL-designed TriG-POD receiver, produced by Moog Broad Reach, and a zenith-pointing choke ring antenna, designed to minimize multipath effects. The TriG receiver is designed to autonomously collect L1, L2, and L5 (not used by DSAC) carrier phase and pseudorange data from up to 24 in-view GPS (Global Positioning System) satellites or other GNSS satellites (also not currently used/needed by DSAC).
Additionally, the DSAC flight software is resident on the OTB's Payload Interface Unit. The payload functions via the USO providing the input frequency to the DU. The DU calculates corrections to the USO signal to produce a stabilized frequency output. This output signal is then used as the reference for the GPS receiver. The GPS system collects the carrier phase and pseudorange data from the GPS constellation (Figure 5) that is later telemetered to the ground for use in the precision clock determination process. The DSAC payload of the USO, DU, and GPS receiver reside in the payload bay of the spacecraft located in the mid-section of the spacecraft bus. The GPS antenna resides on the space-facing panel of the spacecraft, in a configuration that provides an unobstructed view of the GPS constellation during the nominal mission (after launch and early operations has established the nominal attitude). This enables continuous GPS data collection throughout the mission, excepting GPS receiver resets.
Nominal mission operations of DSAC:
The DSAC investigation team consists of two main teams, the TDAS (Technology Demonstration Analysis System ) team (or analysis team for short) and the payload operations team. The TDAS team is responsible for analysis of the collected GPS data to determine the long-term stability of DSAC. GPS orbit and clock solutions are collected by the TDAS system from the JPL GNSS Analysis Center or the IGS (International GNSS Service) via the web. Key TDAS tools used to reconstruct the orbit, relevant dynamic parameters, and the clock performance, include the GNSS-Inferred Positioning System and Orbit Analysis Simulation Software (GIPSY-OASIS) and the Mission-analysis, Operations, and Navigation Toolkit Environment (MONTE). These tools enable the analysis team to perform a combined POD (Precision Orbit Determination) and PCD (Precision Clock Determination) process. The payload operations team is responsible for telemetry collection and commanding, monitoring health and safety, and evaluating the clock technical performance from the telemetry. The telemetry-based assessment will be used in conjunction with the TDAS-based assessment.
Once payload commissioning is complete, the spacecraft is essentially a stable platform for the remainder of the DSAC mission (except for two biannual spacecraft yaw flips). Nominal operations are straightforward with all elements of the DSAC payload powered on, stable, and operating. Data flow to-and-from the payload is depicted in Figure 4. Four types of data are required for the DSAC investigation:
• GPS receiver data (including engineering telemetry and the observables from the GPS constellation) collected nominally on a 10 second interval.
• Clock telemetry (including housekeeping data and clock corrections).
• Platform data, such as attitude, thermal, and other miscellaneous data related to the state of the platform (or externally-sensed temperature of the payload units) that may be useful in correlating signatures in the clock performance to the environment.
• GPS constellation orbit and clock solutions.
The first three data sets are generated on-board and delivered to the DSAC flight team. The GPS constellation orbit and clock solutions used by DSAC are obtained from JPL's IGS Analysis Center (for JPL orbit and clock solutions) and from NASA's CDDIS (Crustal Dynamics Data Information System) space geodesy data archive (for IGS orbit and clock solutions). TDAS is responsible for this interface.
The DSAC-specific GPS receiver data and clock telemetry are collected by the on-orbit computer and stored in a file format on-board. At least once daily, these files are made available for downlink via an automated downlink process. Additionally, a set of platform-related data is downlinked (on a schedule defined by the OTB flight team) and made available to the DSAC flight team. This is an automated process that does not require the DSAC flight team to make any specific data request.
After the data is received on the ground, it is placed on a secure FTP server (by the OTB ground system) within two hours of the end of the pass. Nominally, DSAC data is downlinked on one (single) pass per day, where the DSAC data is prioritized over other non-essential data for downlink during that pass. The DSAC data downlink volume is designed to fit within a single nominal pass; however, a latency of three days is possible in the event of lost passes or other anomalies that interfere with the nominal downlink. Approximately five to six downlink passes may be available per day to downlink all of the data from the OTB (including spacecraft data and data from the payloads). The DSAC ground system retrieves the data from the secure FTP server and delivers it to the DSAC local repository.
The secure FTP server is also the DSAC interface to the OTB uplink process. Command request files are generated by the payload operations team in the form of acquisition schedule XML files. The files allow specification of the execution time for each command as an absolute or relative time. Acquisition schedules are placed on the secure FTP server at least 72 hours prior to the required execution time on the spacecraft. The OTB ground system reviews the command requests to confirm they pose no threat to the platform and are consistent with the expected configuration. DSAC is notified within 48 hours of the required execution time that the command is accepted for uplink. The OTB ground system is responsible for generation of the uplink products and getting them on-board in time for their earliest execution time. Once commissioning is complete, very limited commanding is planned (since changes in the on-board state interfere with the long-term Allan deviation determination and no commanding is required for day-to-day operations).
Figure 13: End to end data flow (image credit: NASA/JPL, Ref. 22)
Key DSAC performance requirements:
The overall DSAC mission needs to determine estimated clock stability (Allan Deviation or AD) of better than 2 x 10-14 at one day assuming that the clock itself is performing at or better than 1 x 10-14 at one day. This can be decomposed into several separate parts:
1) DSAC demonstrates a ground-based stability of better than 3 x 10-15 at one-day in an adiabatic laboratory environment when compared with a hydrogen maser.
2) DSAC demonstrates a spaceborne stability of better than 1 x 10-14 at one-day, where margin has been added to the prior requirement because a changing spacecraft environment (i.e., variable temperature, magnetic fields, radiation) affects clock stability.
3) Verification of DSAC's spaceborne performance introduces an error of no-more than 5 x 10-15 AD at one-day (assuming a perfect input clock) in the overall clock estimation process.
Even though the key requirements specify stability at one day, the project will investigate clock performance on a large range of time scales. On shorter time scales, below 10 seconds, the stability of the USO driving DSAC is the key performance factor. In the 1 – 10 second range, the USO has a stability of better than 2 x 10-13. Fundamentally, the clock control process improves on this stability level over longer time scales and asymptotically approaches a white frequency noise characteristic until it reaches (typically on the order of days) some ‘floor' level performance. Another objective of the DSAC mission is to observe and determine this performance ‘floor'.
If DSAC is able to perform at its prescribed levels, then its utility for one-way radiometric deep-space navigation can be validated by showing that orbit determination performance using the one-way data performs as well as or better than its traditional two-way counterpart. To this end, another key mission requirement is to demonstrate orbit determination uncertainty, in a scenario that represents typical deep-space navigation conditions, of less than 10 m (3σ) using one-way radiometric tracking data with measurement quality, quantity, and schedule characteristics (such as track duration and data gaps) that are operationally similar to that available in deep space navigation.
TDAS (Technology Demonstration Analysis System):
The primary software tools that make up TDAS include GIPSY-OASIS (GNSS-Inferred Positioning System and Orbit Analysis Simulation) and MONTE (Mission-analysis, Operations, and Navigation Toolkit Environment). Each of these are separate NASA-funded capabilities that represent the state-of-the art for Earth orbit determination using GPS data (GIPSY-OASIS) and deep space navigation (MONTE). The DSAC project is leveraging these tools for developing a precision clock determination capability that enables the project to meet its requirements. Given the a priori capabilities of these tools, the DSAC project need only develop specific capabilities needed for PCD and one-way deep space navigation. These enhancements are done in the context of the GIPSY-OASIS and MONTE projects. That is, they are funded by DSAC, but implemented and maintained by their respective development teams. The benefit is the new capabilities receive full operational software pedigrees, and, in addition to DSAC, are available to any other mission that might need them now or in the future. Specific to MONTE, development of precision one-way radiometric measurement modeling, high fidelity clock modeling, and appropriate filter methods is a key feed-forward infusion capability to future missions that would fly DSAC.
The GIPSY-OASIS software is currently used for POD (Precise Orbit Determination) of LEO spacecraft that carry GPS receivers such as gravity missions (e.g., CHAMP and GRACE) and altimetry missions (e.g., TOPEX/Poseidon, Jason-1, Jason-2/OSTM) as well by the geodetic community to determine the precise location of hundreds of ground GPS receivers. It is also used by the JPL IGS Analysis Center to determine the GPS constellation orbits and clocks. These orbit and clock solutions are then combined with additional, independent GPS clock and orbit solutions at the IGS Analysis Coordination Center to produce the IGS GPS orbit and clock solution. Several features of GIPSY-OASIS make it the software of choice to study the performance of the DSAC clock in space, among them:
• Unlike other POD software packages, GIPSY-OASIS does not eliminate the receiver clock error using differenced observables. It is this receiver clock error x (t) that is used to determine the DSAC clock stability.
• GIPSY-OASIS has been used extensively in operations and is continuously upgraded to remain at the state of the art. The DSAC mission will benefit from many of these upgrades.
The MONTE software has been and continues to be developed to serve as the next generation software of choice for deep space navigation and mission design. MONTE is a highly-capable and flexible software tool for trajectory integration, trajectory targeting, orbit determination, and statistical maneuver analysis. MONTE, previously known as the Next Generation Navigation Software, development started in the late 1990's with a charter to upgrade and replace the Fortran 77-based Double Precision Orbit Determination Program (DPTRAJ/ODP) developed in the mid-1960s. MONTE's core computational capabilities are written in C++ and it has a Python-based script/interactive shell user interface. Initially released in 2000 and adopted for operational use in 2006 (by the Mars Phoenix mission), MONTE is currently utilized by JPL flight projects and is continually evolving to include mission- and user-requested enhancements. MONTE has also been released for university research. Though primarily created for deep space navigation and mission design, MONTE is increasingly supporting GPS-tracking Earth orbiters such as GRACE Follow-On and DSAC.
The GIPSY-OASIS software package has been the primary tool for simulating DSAC's expected PCD performance, while MONTE has been used as an independent check of these results. The nominal mission performance results presented in the sequel have been generated by GIPSY-OASIS, and are being used by the project during payload development to ensure DSAC's stability requirements (with associated AD specifications) will be met during operations. MONTE's primary role has been to determine that one-way data derived from DSAC and associated filter designs are sufficient to meet the analog deep space navigation performance requirement (i.e., 10 m (3σ) using a deep space tracking schedule and orbit determination filter design).
ETB (Electronics Test Bed)
The Electronic Test Bed seeks to evaluate and demonstrate a number of new electronic components, processors, and memory devices, enabling in-orbit heritage to be gained on components that may be incorporated into future designs. 24)
• FLexRX (Flexible Receiver): The incorporation of FlexRX, Surrey's next-generation programmable receiver equipment, allows in-orbit test, demonstration, and qualification of a product that will be used on future missions and also sold to third parties. 25)
FlexRX is an enhanced version of a legacy device flown on numerous earlier missions. A FlexRX twin is already in orbit and being tested on the Surrey TechDemoSat-1 launched in July 2014. The performance data collected during the OTB-1 and TechDemoSat-1 missions will help to quickly validate the receiver's readiness for our own operational use and future use by our customers.
The most important technological advancement in FlexRX is its ability to receive data transmitted from the ground at higher uplink rates than the receivers used now. The telemetry commands are received from controllers on the ground, transmitted in the form of radio signals from ground stations. These commands program the satellite to perform various tasks and maintain its overall operation, keeping it safely in orbit.
Satellites can only receive the telecommand data when they have line-of-sight contact with the station. Depending on the orbit of the satellite, this connection time may last just a few minutes before the satellite passes out of sight over the horizon. If the time is brief, the ground station may not be able to transmit all of the necessary commands to the satellite. An obvious solution to this problem is to transmit and receive telemetry at a faster rate.
Surrey has designed the FlexRX to handle incoming data transmissions at a variety of rates. Once we launch OTB-1 in 2016, FlexRX will serve as the primary instrument receiving ground station commands. The goal will be to transmit information from the ground at different rates to confirm the highest stable uplink rate possible for the new receiver throughout the ground station passes. Satellite performance data beamed back to the ground station through a separate instrument will tell us the maximum rate achieved.
Figure 14: Photo of Surrey's FlexRX receiver (image credit: SST-US)
• RadMon (Radiation effects Monitor): The latest generation RadMon device will collect data on the radiation environment in space for correlation with other OTB sensors and for applications in future Surrey missions. RadMon will continuously collect radiation data, providing information on the doses to which OTB components are exposed during the mission. Research by NASA has concluded that electronic microchips begin to suffer performance errors with their memory storage in the 15 to 20 krad TID (total ionizing dose) range. These errors can wreak havoc on the function of a satellite or its subsystems. 26)
Figure 15: Photo of the RadMon sensor (image credit: SST-US)
The RadMon will perform three functions—total dose measurement, particle detection, and dose-rate detection. Dose-rate detection will pick up high and low areas of radiation, helping to spot those regions that have high levels of radiation, such as the SAA (South Atlantic Anomaly). The RadMon payload will allow the project to plot this anomaly.
Figure 16: RadMon will monitor the radiation levels of the SAA (image credit: SST-US)
MSA (Modular Solar Array)
Developed by Vanguard Space Technologies for the U.S. AFRL (Air Force Research Laboratory) in Albuquerque, NM, the MSA will demonstrate flight readiness of a standardized modular approach to solar panels, with the ability for modules to be quickly replaced during final satellite testing prior to launch. Space-qualifying a system with this degree of interchangeability and change-out capability has the potential to reduce schedule delays (and therefore cost) before launch, when there is typically very little remaining schedule margin.
Figure 17: MSA panels accommodation on OTB-1 (image credit: SST-US)
iMESA-R (Integrated Miniaturized Electrostatic Analyzer-Reflight)
Developed by cadets at the USAFA (U.S. Air Force Academy) in Colorado Springs, iMESA-R is a miniaturized sensor that will sample the space environment to find plasma irregularities that may forecast outages in space weather models. While iMESA instruments have flown on previous USAF missions, the variant flying on OTB-1 features a new miniaturized dosimeter design.
Figure 18: Precursor to the iMESA-R variant being flown on OTB-1 (image credit: R.Kiziah)
CUSP (Colorado University Surrey Project)
A joint project between Surrey and the University of Colorado, Boulder, CUSP is a data collection and storage electronic test bed experiment built by students using off-the-shelf components.
In addition to carrying the above five payloads, OTB will test a Terminator Tape Deorbit Module—a device developed by TUI (Tethers Unlimited Inc.) of Bothell, Washington, USA, to deorbit the OTB satellite at the end of its planned mission. TUI used their Cubesat tether design as the basis for this new, larger Small Satellite tether. The OTB mission will be the first Surrey spacecraft to fly a tether-based deorbit system. This passive de-orbit device is an ideal mass, power, volume, and cost-efficient solution to complement the business case closing objectives of the OTB mission; any typical fluid or solid-based propulsion device would impact the payload carrying capabilities of the satellite platform, in addition to requiring active control at end of life and additional redundancy and safety mechanisms.
Figure 19: Prototype version of the Terminator Tape Deorbit Module (image credit: TUI)
Figure 20: Overview of the planned on-orbit phases (image credit: SST-US, Ref. 7)
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9) "General Atomics Acquires Assets of Surrey-US, LLC," General Atomics, 13 November 2018, URL: http://www.ga.com/general-atomics-acquires-assets-of-surrey-us-llc
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14) "NASA Selects Surrey Satellite US for Flight of Deep Space Atomic Clock Payload," SSTL, June 27, 2013, URL: http://www.sstl.co.uk/Press/NASA-Selects-Surrey-Satellite-US-for-Flight-of-Dee
15) "Deep Space Atomic Clock (DSAC)," NASA, Dec. 2011, URL: http://www.nasa.gov/mission_pages/tdm/clock/clock_overview.html
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18) John D. Prestage, "Next Generation Space Atomic Clock, Space Communications and Navigation (SCaN) Technology," NASA/JPL, 2011, URL: http://scpnt.stanford.edu/pnt/PNT11/2011_presentation_files/19_Prestage-PNT2011.pdf
19) "Deep Space Atomic Clock (DSAC)," NASA, Oct. 9, 2012, URL: http://www.nasa.gov/directorates/heo/scan/engineering
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21) Todd A. Ely, Jill Seubert, Julia Bell, "Advancing Navigation, Timing, and Science with the Deep Space Atomic Clock," SpaceOps 2014, 13th International Conference on Space Operations, Pasadena, CA, USA, May 5-9, 2014, paper: AIAA 2014-1856, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2014-1856
22) Todd A. Ely, David Murphy, Jill Seubert, Julia Bell, Da Kuang, "Expected performance of the Deep Space Atomic Clock mission," Proceedings of the 24th AAS/AIAA Space Flight Mechanics Meeting, Santa Fe, NM, USA, January 26 –30, 2014, paper: AAS 14-254
23) Todd A. Ely, Jill Seubert, "One-way radiometric navigation with the Deep Space Atomic Clock," 25th AAS/AIAA Space Flight Mechanics Meeting, Williamsburg, VA, USA, January 11-15, 2015, paper: AAS 15-384
24) "OTB Payload Profile: RadMon to Measure Radiation in Low-Earth Orbit," SST-US, 2014, URL: http://www.sst-us.com/blog/october-2014/otb-payload-profile-radmon-to-measure-radiation-in
25) "OTB Blog Series: FlexRX Receiver to Gain In-Orbit Heritage," SST-US, 2015, URL: http://www.sst-us.com/blog/may-2015/otb-payload-profile-flexrx-receiver-to-gain-in-orb
26) "OTB Payload Profile: RadMon to Measure Radiation in Low-Earth Orbit," SST-US, 2014, URL: http://www.sst-us.com/blog/october-2014/otb-payload-profile-radmon-to-measure-radiation-in
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).